I think I can understand what you are trying to do but it doe's raise some questions.
If you are not sure of the speed limit, then you need to work on your observation, what was the last speed sign passed?
If this is the problem, this suggests your powers of observation are not very good.
One way of helping to develop this is to give yourself a running commentary as you drive along, have a look on You Tube there is a video that gives a good explanation, and demonstration.
https://www.youtube.com/results?sear...+while+driving
If you don't trust the technology, think of it this way, if the speed camera is faulty, what speed is it going to trigger at?
If it is faulty, then who knows?
You might as well drive past at the speed limit or a speed appropriate to the conditions, and that is what the majority of other drivers will expect you to do.
Most fixed speed cameras, will be checking the speed electronically, and physically.
There will be a pattern of paint lines on the road which will be included in the photographs produced by the camera, by knowing the time between the photographs the distance covered in the time between pictures can be measured, from this the actual speed can be calculated.
The other fixed camera type has two sensors in the road, and the camera measures the time taken between sensors to arrive at speed.
As you can see the risks of error are quite small.
Finally, if you were to be mistakenly caught speeding for the first time, life will not end, you will get a fine and points on your licence.
The problems only start if you repeatedly offend.